Terahertz waves are electromagnetic waves generally having a frequency of 0.1 THz to 10 THz, and are expected for basic fields such as physical properties, electrospectroscopy, bioscience, chemistry and pharmaceutical science and for applied fields of atmospheric environmental assessment, security, material test, food test and communication.
As a device for oscillating terahertz waves, it has been developed a backward wave oscillator (BWO) and photomixing in several hundreds GHz band and free electron energy laser, p-Ge laser and quantum cascade laser (QCL) for 1 THz or more. These devices, however, have problems in the miniaturization and improvement of output power.
On the other hand, it has been recently developed a device for oscillating a wide-band terahertz wave with an optical switch or rectification using a femtosecond laser as a light source and applied for time domain spectroscopy (TDS) or the like
In addition to this, for generating terahertz wave using non-linear optical crystal such as LiNbO3, it has been known methods of utilizing quasi phase matching (QPM) and of utilizing phonon polariton. Such methods are expected for applications requiring sources generating the terahertz wave having high temporal and spatial coherency.
It is described, in Japanese Patent Publication No. H9-146131A, device of generating terahertz wave using a y-plate or z-plate of an LiNbO3 substrate. That is, pump wave is irradiated onto the substrate from a light source, and at the same time, idler wave is irradiated onto the same substrate. The pump wave (frequency ω1), idler wave (frequency ω2) and polariton (terahertz wave: frequency ωT) satisfy law of conservation of energy (ω1=ω2+ωT) and law of conservation of momentum (noncollinear phase matching condition: k1=k2+kth), so that polariton stimulated scattering is observed. In this case, due to the scattering characteristics, the frequencies of the idler wave and terahertz wave are decided depending on the angles α and θ of the pump wave with respect to the optical axis.
According to the method, typically, the phase matching condition is satisfied when an angle α of wave vector k1 of the pump wave and wave vector k2 of the idler wave is 0.5° to 1° and the terahertz wave was then oscillated (wavelength of 100 to 300 μm, frequency of 3 THz to 1 THz) at a high efficiency. Further, it is described that the terahertz wave is oscillated at an angle of 65 to 66° with respect to the idler wave. In the case that a y-plate is used, the crystal orientation is different from that of the z-plate. As a result, the pump wave and idler wave propagate on a plane perpendicular to the substrate surface in an angle α to generate terahertz wave at an angle θ with respect to the pump wave.
However, (1) the crystal has a refractive index as high as 5.2 with respect to sub-milli wave (terahertz wave) so that total internal reflection occurs between the crystal and air. It is thus impossible to emit the terahertz wave into the air both in the cases of the y-plate and z-plate. (2) Optical loss in the crystal is large. For example, the optical intensity of the terahertz wave is reduced to about 0.1 percent with respect to a propagation distance of 3 mm of the terahertz wave. For these problems, according to Japanese Patent Publication No. H09-146131A, a grating is provided on a side face 5 of the substrate for emitting of the terahertz wave into the air at a high efficiency.
According to Japanese Patent Publication No. 2002-72269A, an exciting laser light having a single frequency is irradiated and an idler wave having a single frequency is used for optical injection to generate terahertz wave having a high output power and whose spectrum line width can be reduced. A silicon prism is, however, used for outputting the terahertz wave.
As described above, according to the prior devices, when terahertz wave is oscillated based on parametric oscillation, most of the generated wave is absorbed within a crystal. In addition to this, since the refractive index of the crystal is considerably higher than that of air, it is not possible to emit the generated terahertz wave to the outside of the crystal without providing a silicon prism or grating on a surface of the crystal. The intensity of the terahertz emitted to the outside of the crystal is too low for actual applications and it is required an additional step of providing the prism or grating on the surface of the crystal. Further, the provision of the prism or grating inevitably causes propagation and reflection losses due to them.
On the other hand, according to prior arts of generating visible and infrared rays, it has been tried to subject an optical device to fine processing to form a three-dimensional patterned structure having an interval or period smaller than a wavelength of the light on the surface of the optical device, so as to provide the function of preventing reflection on the surface (Japanese Patent publication No. 2002-287370A; Japanese Patent Publication No. 2004-521329A; Japanese Patent Publication, No. 2003-177210A; Japanese Patent Publication No. 2010-020120A; “OPTICS LETTERS” Vol. 24, No. 20, October 15, page 1422 (Optical Society of America); ┌KONICA MINOLTA TECHNOLOGY REPORT┘ Vol. 2 (2005), pages 97 to 100 “Polymeric Wide-band Wave Plate Produced via Nanoimprint Subwavelength Grating”; “Synthesiology”, Vol. 1, No. 1 (2008), pages 24 to 30, “Challenge for production of high functional optical devices at low costs - - - Realization of sub-wavelength periodic structure by glass imprint method”).